Lower sertraline plasma concentration in patients co‐medicated with clozapine—Implications for pharmacological augmentation strategies in schizophrenia

Abstract Augmentation of antipsychotic treatment with antidepressants represents a common and beneficial treatment strategy in patients suffering from schizophrenia. Combining clozapine and the selective serotonin reuptake inhibitor (SSRI) sertraline represents a clinically important strategy in patients with therapy‐resistant schizophrenia, but there is limited knowledge about mutual pharmacokinetic interactions. In the present study, we assessed the impact of clozapine on sertraline plasma concentrations. Based on a therapeutic drug monitoring (TDM) database, sertraline plasma concentrations were compared between two groups: patients receiving a combined treatment with sertraline and clozapine (N = 15) and a matched control group receiving sertraline but no clozapine (N = 17). Group differences with respect to raw and dose‐adjusted concentrations were assessed using nonparametric tests. Comedication with clozapine was associated with 67% lower median sertraline plasma concentrations (16 vs. 48 ng/mL; p = .022) and 28% lower median dose‐adjusted plasma concentrations (C/D; 0.21 vs. 0.29 (ng/mL)/(mg/day); p = .049) as compared to the control group. Scatter plots revealed a complex relationship between the dosage of clozapine and dose‐adjusted sertraline concentrations composed of an initial decrease at clozapine doses below 300 mg, an increase between 300 and 600 mg and a final decrease at 800 mg which was best modeled by a third order polynomial term. Cotreatment with clozapine may lead to reduced sertraline plasma concentrations which may be explained by clozapine‐induced gastrointestinal hypo‐mobility already present at low doses and cytochrome P450 3A4 inducing properties at high clozapine doses. For this drug combination, clinicians should consider TDM to confirm therapeutically effective plasma concentrations of sertraline.


Abstract
Augmentation of antipsychotic treatment with antidepressants represents a common and beneficial treatment strategy in patients suffering from schizophrenia. Combining clozapine and the selective serotonin reuptake inhibitor (SSRI) sertraline represents a clinically important strategy in patients with therapy-resistant schizophrenia, but there is limited knowledge about mutual pharmacokinetic interactions. In the present study, we assessed the impact of clozapine on sertraline plasma concentrations.
Based on a therapeutic drug monitoring (TDM) database, sertraline plasma concentrations were compared between two groups: patients receiving a combined treatment with sertraline and clozapine (N = 15) and a matched control group receiving sertraline but no clozapine (N = 17). Group differences with respect to raw and doseadjusted concentrations were assessed using nonparametric tests. Comedication with clozapine was associated with 67% lower median sertraline plasma concentrations (16 vs. 48 ng/mL; p = .022) and 28% lower median dose-adjusted plasma concentrations (C/D; 0.21 vs. 0.29 (ng/mL)/(mg/day); p = .049) as compared to the control group.
Scatter plots revealed a complex relationship between the dosage of clozapine and dose-adjusted sertraline concentrations composed of an initial decrease at clozapine doses below 300 mg, an increase between 300 and 600 mg and a final decrease at 800 mg which was best modeled by a third order polynomial term. Cotreatment with clozapine may lead to reduced sertraline plasma concentrations which may be explained by clozapine-induced gastrointestinal hypo-mobility already present at low doses and cytochrome P450 3A4 inducing properties at high clozapine doses. For this drug combination, clinicians should consider TDM to confirm therapeutically effective plasma concentrations of sertraline. ucuronosyltransferase-2B7. 16 Sertraline's elimination half-life is between 22 and 36 hours, 11 and plasma concentrations in a range between 10 and 150 ng/mL are considered as therapeutically effective. 11 Only few studies addressed pharmacokinetic or pharmacodynamical interactions between clozapine and sertraline. 17,18 Specifically, a tendency toward higher plasma concentrations of clozapine in patients comedicated with sertraline compared with clozapine monotherapy has been described in a study controlling for smoking status. 17 To the best of our knowledge, however, there has been no study addressing the effects of clozapine on sertraline plasma concentrations in patients treated with the combination of clozapine and sertraline. Therefore, we conducted a retrospective analysis of sertraline plasma concentrations from a therapeutic drug monitoring database comparing sertraline concentrations in patients with and without co-medication with clozapine.

| MATERIAL S AND ME THODS
KONBEST, a web-based laboratory information management system for therapeutic drug monitoring laboratories, 19 11,20 were excluded from the analysis. Among patients with more than one measurement of sertraline plasma concentration, we selected the most recent measurement. 21 Hence, TDM data of 794 in-and outpatients with a broad spectrum of psychiatric diseases were eligible for analysis. Based on this sample, we considered two groups: a group of patients receiving sertraline with co-medication with clozapine (SERT CLZ , N = 15, i.e., all patients receiving clozapine and considered eligible according to the abovementioned criteria) and a control group receiving sertraline, but no clozapine (SERT, n = 17). The control group was extracted out of the remaining 779 patients who did not receive clozapine, by a sequential matching procedure. For each subject of the SERT CLZ group, we first selected all subjects with the same gender. From those subjects, we sequentially selected the subjects with the lowest age difference, lowest weight difference, same status of smoking, same status of caffeine consumption, and lowest dose difference (in this order) clozapine, pharmacokinetics, schizophrenia, sertraline, therapeutic drug monitoring as compared to the respective subject of the SERT CLZ group. When no subject of the preliminary control group remained at a certain matching step (e.g., a few subjects were identified having the same gender, the same age, and weight, but all have them had a different smoking status as compared to the subject of the SERT CLZ group used in the current iteration of the algorithm), all subjects of the previous matching step entered the final control group (SERT). Subjects who entered the final control group were not eligible anymore as control subjects for the next subject of the SERT CLZ group (i.e. the next iteration of the algorithm).
Demographic and clinical characteristics of the sample are provided in Table 1. Group differences concerning sociodemographic variables were assessed using Mann-Whitney U-tests or chi-squared tests (for the comparison of frequencies).

| Quantification of sertraline
Blood samples were drawn just before drug administration (i.e.

FDA (US Food and Drug Administration) guidance (US Food and
Drug Administration, 2018), 24 and ICH (International Conference on Harmonization) requirements. 25 The laboratory regularly runs internal quality controls and participates in external quality assessment schemes by INSTAND (Düsseldorf, Germany, www.insta ndev.de).
Inaccuracy and inter-and intraday inaccuracy were evaluated at sertraline concentrations of 300, 100, and 5 ng/mL, respectively.
Lower limit of detection (LOD) and limit of quantification (LOQ) were 3.6 and 7.2 ng/mL, respectively.  In order to evaluate the relationship between the clozapine dose and dose-adjusted drug concentrations of sertraline, we generated scatter plots and assessed the fit of linear and nonlinear regression models. Among the nonlinear regression models, we tested second and third-degree polynomials.

| RE SULTS
The demographic data of the study groups are displayed in Table 1.
No significant differences were found between the groups regarding daily dosage of sertraline, age, weight, sex distribution, caffeine consumption, smoking, or suspected nonadherence (all p-values >.05).
Plasma concentrations of sertraline were lower in the group comedicated with clozapine compared with the control group (p = .022, Mann-Whitney U-test, for detailed statistics see Table 2); accordingly dose-adjusted drug concentrations were lower in the group co-    Table 3).
Since the final decrease in the dose-adjusted sertraline concentration was only based on one observation (at 800 mg clozapine), we also repeated all regression analyses after removing this data point.
This additional set of exploratory analyses confirmed again that a linear model was not the most suitable to explain the data. As to be expected, in this case, a second-degree polynomial exhibited the best model fit (see Table S1 and Figure S1).

| DISCUSS ION
The augmentation of an antipsychotic treatment by adding an antidepressant drug represents a common and potentially beneficial strategy for targeting negative and depressive symptomatology in patients suffering from schizophrenia. 7,28 The combined treatment with clozapine and sertraline is a clinically particularly meaningful combination due to clozapine's superior potency with respect to treatment-resistant schizophrenia and sertraline's favorable efficacy and safety profile among different antidepressant drugs. While previous studies focused on the effect of sertraline on plasma concentrations of clozapine, the present study shows-to our knowledge-for the first time that a cotreatment with clozapine and sertraline is associated with significantly lower plasma concentrations of sertraline.
In 20% of the co-medicated patients, plasma concentrations were even below the therapeutic reference range, whereas we did not observe subtherapeutic concentrations in the control group. In accordance with our findings, Srisuma and coworkers 29  One potential mechanism might be clozapine's inhibitory effect on gastrointestinal motility, particularly delayed gastric emptying, which is attributed to its antagonistic properties at muscarinic cholinergic, serotonergic, and histaminergic receptors. 30 Clozapine-induced gastrointestinal hypomotility typically manifests as constipation, but can also reach life-threatening stages. [30][31][32] Importantly, decreasing the rate of gastric emptying leads to a reduced bioavailability of most orally administered drugs. 33 Since for a majority of drugs, their absorption in the stomach is rather low, a longer dwelling time in the stomach reduces the proportion of unchanged drug available for absorption in the small intestine. 33,34 Consequently, the here reported lower sertraline plasma concentra- first evidence for a CYP3A4-inducing effect of clozapine which was confirmed both for CYP3A4 mRNA and activity level. 37 However, this effect was only observed for rather high clozapine concentrations starting around the upper limit of the therapeutic reference range. Accordingly, this phenomenon may underlie the second dip of the dose-adjusted sertraline concentration observed at the clozapine dose of 800 mg in our data. However, this conclusion requires caution as we only collected one observation at this dose. Between these two dips of the sertraline concentration, we observed an increase at clozapine doses between 300 and 600 mg. We assume that this increase is related to the fact that both drugs are substrates of CYP isoenzymes 2C19 and 3A4 and accordingly act as competitive inhibitors.
The present finding may raise the question whether other antidepressant drugs should be preferred over sertraline when considering an augmentation strategy to an antipsychotic treatment with clozapine. In general, SSRIs represent first choice agents due to their favorable properties regarding efficacy and toxicity. Moreover, they exhibit superior efficacy for the treatment of obsessive compulsive symptoms, which are frequently observed in patients with schizophrenia, particularly patients under a treatment with clozapine. 38,39 Among the different SSRIs, fluoxetine, and paroxetine may increase clozapine levels due to their CYP2D6-inhibiting properties. 18,40 Fluvoxamine is a potent CYP1A2 inhibitor and may be used to achieve therapeutically effective plasma concentrations in smokers. 41 For the newest SSRIs, the racemic compound citalopram Once this concentration is known, it may guide dose adjustment when new pharmacokinetic drug-drug interactions are expected to emerge (e.g., clozapine is added to the treatment with sertraline) or pre-existing interactions are expected to disappear (e.g., clozapine treatment is discontinued, while sertraline treatment is continued).

| Limitations
As we retrospectively conducted this analysis, patient information is likely less reliable than in prospective studies. A major limitation of the present study is the limited number of patients. Therefore, the quantitative estimates of clozapine's impact on sertraline plasma concentrations have to be considered as rather uncertain. Another limitation is the non-normal (right-skewed) distribution of plasma levels requiring nonparametric statistical tests, which is however, a common finding related to TDM studies ( 46,47 ). As a further limitation, plasma concentrations of desmethylsertraline, the main metabolite of sertraline, were not available. Moreover, many clinical parameters such as the duration of illness, the clinical phenotype, adverse effects, comorbidities, renal function parameters as well as the duration of prior treatment with sertraline and the co-medication were not available. In addition, differences in plasma concentrations of sertraline may be also associated with polymorphisms of the corresponding CYP isoenzymes which we, however, did not assess in the present study. Moreover, we cannot definitely exclude the possibility that the findings might be in part explained by differences in adherence. However, as we show in Table 1, the proportion of patients for whom TDM was ordered due to suspected nonadherence did not significantly differ between groups. Therefore, we consider differences in adherence as unlikely to explain the findings obtained in the SERT CLZ group. Finally, it has to be stated that the use of clozapine is generally limited to treatment-resistant schizophrenia, that is, the here reported findings are only relevant for a subset of patients.
However, therapy resistance is highly prevalent in schizophrenia affecting around one third of patients. Nevertheless, clozapine is considered underused in many different regions in the world. 48  Abbreviations: Adj., adjusted; CLZ, clozapine; df, degrees of freedom; Poly2/3, second/third-degree polynomial (note that the linear model is a first-degree polynomial); SE, standard error.